1. Field of the Invention
This invention pertains to the general field of topographic profilometry and, in particular, to a technique for improving the process of stitching multiple maps obtained by independent profilometric measurements of a sample to form a composite map. More particularly, the invention relates to a stitching procedure for interferometric profilers.
2. Description of the Prior Art
It has become common practice in interferometric profilometry to combine profiles of adjacent sections of a sample surface to form a composite profile. Each section of the surface is profiled independently, thereby making it possible to retain with each successive measurement the optical resolution provided by the field of view corresponding to the size of the section being sampled. This process is commonly referred to in the art as a “stitching” procedure.
The x,y translation of the sample stage with reference to the interferometric objective may introduce tip, tilt and offset variations between measurements that require adjustments to the profile data in order to avoid errors. Such corrections are typically implemented by using information derived by measuring overlapping regions of adjacent sections of the sample surface. Another source of errors lies in the nonlinearity and lack of repeatability of the vertical motion of the scanner employed to produce the interferometric fringes that allow profiling of the sample surface. That is, the absolute position of the interferometric objective at the origin and during the step sequence of the scan may vary between measurements because of inertial and other effects, such that separate profiles cannot be reliably combined without appropriate correction. This is again accomplished by taking measurements of adjacent sections with overlapping regions, so that information relating to both sections can be obtained and used to normalize the profiles of the sections to a common reference surface.
For example, commonly owned U.S. Pat. No. 5,987,189, No. 5,991,461 and No. 6,185,315 describe techniques for correcting stitching errors introduced by the scanner and the x,y translation of the sample (or of the interferometric objective) between measurements of the various surface sections of interest. In essence, successive measurements of adjacent sections of the sample surface are taken by sequentially placing them within the field of view of the instrument and independently profiling each section by conventional interferometric procedures, such as phase shifting or vertical scanning interferometry. The x-y translation of the microscope between successive measurements is carried out by maintaining a region of overlap between sections, so that spatial continuity is retained between measurements. The height data generated for each section are then combined to form a larger image corresponding to the entire surface tested and discontinuities and/or errors introduced by the x-y translation process are corrected by normalizing the overlapping portions to a common reference plane. This is achieved by fitting a plane equation through each set of measured heights in the overlapping regions, and the tip, tilt and offset of one of the fitted planes are used to correct the height data of adjacent sections and produce matching overlapping profiles. The measured height data for the balance of the adjacent section are then also corrected by the same difference in tip, tilt and offset to obtain a continuous normalized image.
Other stitching techniques in the art also rely on overlapping measurement regions to provide the information necessary to normalize all data and provide a consistent composite map of the sample surface. This limitation requires redundancy of measurements and does not allow reliable stitching of data collected from sample sections separated by intervening unprofiled regions. While x,y positioning errors may be reduced to acceptable tolerances by using high-precision translation stages, similar precision hardware is not yet available to reduce to acceptable levels the z-axis (offset) errors produced by scanner-motion nonlinearity and non-repeatability. Therefore, no stitching method has yet been disclosed in the art that avoids the need for taking measurements of overlapping regions of profiled sections in order to obtain correlating information that can be used to calculate appropriate correction and produce reliable composite profiles. Thus, a stitching process that overcomes these shortcomings would be very desirable.
Another limitation of current profiler technology resides in the measurement of temporal changes occurring in sample surfaces or structures of interest. For example, the objective of a measurement may be to identify changes in the topography of a membrane as a result of its interaction with a chemical agent. That is obviously accomplished by profiling the sample at different times and comparing the results to calculate changes. Using prior-art profilometric techniques, these changes can be measured accurately only when at least a portion of the sample within the field of view remains unchanged. That is, the measurements related to the unchanged portion are used to provide a common reference from which changes can be calculated. Accordingly, it would also be very desirable to develop a process that allowed calculation of temporal surface changes when the entire sample within the field of view varies with time.
Commonly owned pending application Ser. No. 09/888,826 discloses the use of an optical reference signal dedicated to providing a full history of the scanner motion during interferometric measurements, thereby allowing a determination of scanning errors at each step. The reference signal is independent of the fringe information collected for measurement purposes during the scan. The concept may be implemented by utilizing an additional light source with the same scanner used for the measurement (with the same or a different detector), so that the optical path difference (OPD) varies in synchronization with both the reference-signal and the data-collection procedures. Alternatively, a high temporal-coherence filter and/or a reduced numerical-aperture objective may be used with the same light source and optical path used for the interferometric measurement.
The present invention is based on the realization that the reference signal described in the referenced patent application provides a solution to the problem of stitching interferometric profiles without requiring spatial continuity between them. Accordingly, this disclosure also enables stitching without normalization of the profile data to a common reference plane and without recourse to other equivalent solutions.